Indeed the word that in Old English meant "grit, sand, earth" (gréot, Old English Dictionary) and in Middle English meant "Sand, gravel, small stones" and similar items ("gret, n.(3)," Middle English Dictionary) developed another meaning as a character quality in the 19th century. How did it do this? Like many figurative uses, the answer is unclear. The general association may be between roughness or hardness of stone and of personal character.
First, grit was understood to be coarse or rough, and thus suitable for grinding. Yes, that roughness may be made by relatively small fragments of rock, but the association between grit and texture goes back very far. Here is the OED, "grit, n.1.," def. 2a, with an early example and one from the late 18th century:
a. Coarse sandstone, esp. of the kinds used for millstones and grindstones; gritstone.
c1275 Serving Christ 67 in Old Eng. Misc. 92 Me graueþ þis gode in greote and in ston. [graven can mean to carve or engrave]
1784 J. Belknap Jrnl. 29 July in Tour to White Mts. (1876) 20 Grindstones are found at Fryeburg and at Amariscogin, of a fine grit, and hard.
Similarly, definition 4 mentions the relative texture of stone:
- The grain or texture of a stone, in respect of fineness, coarseness, etc.
?1530 J. Rastell Pastyme of People sig. Ciiv These stonis at stonehenge be all of one gryt without chaunge of colour or vayne.
1776 G. Semple Treat. Building in Water 40 The Grit or Grain of it greatly resembled that of a Millstone.
If the coarseness of fineness of the grit provided one way for masons to use stone and early naturalists to read it, these qualities may have inspired early figurative use applied to people. Here is definition 5a:
- colloquial a. Originally U.S. slang. Firmness or solidity of character; indomitable spirit or pluck; stamina. to be clear grit, hard (etc.) grit: to have genuine spirit or pluck. to be the grit: to be the ‘right sort’, the genuine ‘article’.
1825 J. Neal Brother Jonathan III. 386 Proper fellow he was too; 'cute enough, I tell you—sharp as a razor—clear grit.
At first I parsed the phrase as "razor-clear grit," but looking at the original printing, that appears to be an em-dash and not a hyphenated word: "sharp as a razor --- clear grit." "Clear grit" is a distinct quality. The clear here means "pure" or "unadulterated" (OED, "clear, adj., adv. and n.," def. 25a.). Like many figurative expressions, "clear grit" appears with little trace of the thought-process behind it. The association with firmness or texture is educated guesswork.
Second, going back any further in usage yields little more clarity.
Here is "To Dick Slider" in the 28 November 1804 edition of the Litchfield Monitor:
For here's the little (bachelor) gentleman, as Gauger calls him, that knows as much as you and your father and your grandfather, and is most as old; in fact he an't any thing but one solid serap and morsel of clear grit and learning. (Clear grit italicized in original)
An article in the 14 November 1816 edition of The New York Courier repeats the phrase:
The New-York Electors are all clear Grit; Monroeites, to the back bone.
Even these earlier writers are using the phrase as if its meaning would be understood or at least parsed by their readers. They suggest the phrase may have been circulating for a few decades prior to the OED's first mention, but give little further clue of meaning or history.